VSEPR

from http://www.uwosh.edu/faculty_staff/xie/tutorial/vsepr.htm#live

VSEPR Theory

Valence Shell Electron Pair Repulsion

In General Chemistry, we learned how to use VSEPR theory to predict the shapes of molecules. This theory predicts that bonding (sigma bonds only) and non-bonding electron pairs in a molecule will adopt a geometry in which the distance between the electron pairs is maximized from one another in order to minimize the repulsions. This will result in a molecular geometry with the lowest possible energy. The theory also allows us to predict which hybridization the central atom takes in bonding to other atoms.

To start, we need to know the Lewis structure of a molecule. Then we count how many pairs of electrons (triple bonds are counted as one pair) are around the central atom. If there are two pairs of electrons, they must be positioned 180° apart from each other and the shape is therefore linear. Three pairs are best positioned 120° apart and the shape is thus trigonal planar. Here the shape is referred to include the non-bonding electron pairs. For the shape of a molecule without counting non-bonding electron pairs, make a normal prediction then look at the molecule without non-bonding electrons showing. Four pairs of electrons are best positioned as tetrahedral shape. Depending upon the number of non-bonding electron pairs, the shape of the molecule not counting non-bonding electron pairs can be: a) tetrahedral (no non-bonding pairs); b) trigonal pyramidal (one non-bonding pair); or c) "bent" or "V" (two non-bonding pairs). For five pairs of electrons, the shape is predicted to be trigonal bipyramidal. Last, the octahedral is the shape predicted for six pairs of electrons.

In this tutorial, you will see some "live" molecules on your computer screen. You can rotate each one to see their shape. After that, you may want to test yourself on the following questions:

1. How many pairs of electrons (sigma bonds and non-bonding pairs) are around the central atom?

2. What is your predicted shape of the molecule including the electron pairs? What is the predicted shape of the molecule without the electron pairs?

3. What is the hybridization involved in the bonding of the central atom?

When you are ready to check the answers, click on "Check the Answers" located near each molecule.